1. Field of the Invention
This invention relates to a rotary table with an index apparatus for indexing the rotary table.
2. Discussion of the Prior Art
A conventional index apparatus for indexing a table of a machine tool is disclosed by Japanese Patent Application Laid Open No. 24380/1973. The indexing apparatus is provided with a table base by which a rotary table is supported for rotational movement about a vertical axis, and a piston member which is arranged in the table base to be hydraulically moved in a vertical direction while the rotational movement thereof is prohibited. In the index apparatus, three coupling gears are arranged for accurately indexing the rotary table. Namely, the first coupling gear mechanically connected to the rotary table is coaxially arranged with respect to the second coupling gear fixed to the table base, and these coupling gears are formed with coupling teeth at their upper surfaces. Further, the third coupling gear is fixed to the above-mentioned piston member, and is formed with coupling teeth at its lower surface to be engaged with the first and second coupling gear when the piston member is moved downward. The third coupling gear is composed of an inner gear portion fixed directly to the piston member and an outer gear portion which is connected to the inner gear portion via an elastic member which allows the vertical movement of the outer gear portion.
Before the rotary table is rotated, the piston member is initially moved upward to break off the engagements between the first and second coupling gears and the third coupling gear, and the rotary table is then rotated by a predetermined angle. The piston member is then moved downward so that the inner and outer gear portions of the third coupling gear are meshed with the second and first coupling gears, respectively, whereby the rotary table is slightly rotated. With this operation, a workpiece mounted on the rotary table is accurately positioned to a desired angular position for machining.
As mentioned above, in the conventional indexing apparatus, the third coupling gear is divided into the inner and outer gear portions which are connected with each other via the elastic member. The conventional index apparatus therefore has the following advantages:
1) The rotary table can be accurately positioned to a desired angular position, because the assembling errors of the first, second and third coupling gears can be absorbed by the deformation of the elastic member.
2) The positioning accuracy in the vertical direction of a workpiece can be improved, because the rotary table is indexed without any vertical movement.
However, in the conventional index apparatus, the rotary table becomes unstable in some cases, because the periphery of the rotary table is not supported by the table base in the vertical direction. Accordingly, when an eccentric load or unbalanced load acts on the rotary table due to the shape or the mounting position of the workpiece, or when a large cutting force acts on the workpiece during machining operations, the rotary table is partially lifted from the table base, whereby the machining accuracy deteriorates.